Would you be interested in writing a cow-pony transformation fic? I think it would be interesting to see Big Macintosh on the receiving end of a potion/poison joke mishap, growing fatter and developing a large, milky udder overnight. In an attempt to relieve pressure from their brother so he can move again, Applejack and Apple Bloom drink straight from the source, unknowingly fattening and becoming addicted to his milk over time.

Mac groaned as the poison joke started to flood through his system. The glass of “cider” clattered to the ground, forgotten, as his every limb started to feel heavier than a bale of hay.

At first, the effects seemed like normal poison oak - second by second, his cheeks started puffing out until he could see either one in his peripheral vision. But it was only a few seconds later that he realized that they’d stopped swelling - and that they weren’t sore, itchy, or burning. Meanwhile, the space under his belly had started to get heavier and heavier.

Wait - what? He glanced down hurriedly between his legs, only to see his belly starting to bulge out down beneath him, thickening with rolls of fat. Each one of his legs was thickening, too, until just picking one up into the air felt like a herculean task. Big Mac panted in the middle of the barn, panting as he felt the fat filling in his every curve, and several places where it had to create new ones. What was worse was that certain discoloration had started spreading out across his fur, leaving great pale blotches on his coat that were more than reminiscent of a cow’s spots.

By the time he’d made it to the end of the stall, his growing hindquarters had gotten so wide as to touch the walls on both side of him. Big Mac groaned, trying to throw his now-massive weight forward out of the stall and onto the floor of the barn, but though the stall shook, his fat butt wouldn’t budge.

He finally gave up with a huff, sweat streaming down his forehead as he thumped down to the ground. The floorboards shook and creaked under his massive weight, though his belly ballooned out under him, catching him as he fell into what felt like a big, soft pillow. His gut jiggled and gurgled beneath him, his fat cheeks and double chin jiggling along with it as he lay, near-completely immobile, on the floor.

“Nope,” he said sadly, glancing down at his fat, motionless form. Hollering to the house wouldn’t get help this late at night, and with this much fat on him, he wouldn’t be able to move an inch. Better to wait, and hope that Applejack turned up in the morning.

So he closed his eyes and slept, snoring atop a pillow of his own fat. And that was how they found him the next morning, a big blob of fat in the middle of the barn so big that the sides of the stall had snapped and crashed to the floor in the night.

“Whoa, nelly!” Applejack cried upon seeing him. “Mac, what happened?”

He just looked at her. “Poison joke,” he said, his triple-chin rippling with every syllable.

“Darn it,” she said. “You’re just downright huge! We ain’t gonna fit you in no tub like that.”

And he was - huge, that is. With a belly so big that it pushed his stubby little legs a good foot off the ground, Mac was well and truly fat. He’d always been a big stallion, but now Applejack had to crane her neck even further to look him in the eye - and that was not something that either of them cherished.

“Nope,” he said, again sadly.

“Well,” she said, starting to walk around him, “we’ll have to do something with you. What’s this?”

Big Mac struggled to see the bottom of his fat, flabby stomach, but couldn’t see much more than the gut itself.

“Looks like…” Applejack’s nose wrinkled. “An udder?” She poked it with a hoof, and a thick stream of milk squirted out.

“Now that’s somethin’ you don’t see every day,” she said. “Lemme nab a taste.”

Big Mac groaned under his breath as Applejack leaned in close and took his udder gently in her mouth. She sucked on it for a moment, at first gently, then greedily.

“Woo-whee!” she said, pulling away at last. There was a circle of white around her muzzle, and an odd look in her eyes. “Now that there’s some fine milk. Maybe if we get enough out of you, you’ll be small enough to move again!”

“Eeyup,” Mac said. Seemed as good a plan as any.

But Applejack was already back beneath him, latched onto the udder that was poking out from under his bulk. He groaned again in pleasure as she sucked at his teat, his stubby legs wiggling in the air as Applejack drank and drank and drank.

Finally, after what felt like a quarter of an hour, Applejack pulled away, grunting. “High-quality milk,” she said, gasping. “High-quality milk.”

There was now a wider curve to her flank, a bit of give to her neck. Her belly was a bit more rounded than before, and in fact seemed a bit soft, if not downright jiggly. Big Mac said nothing, though; he figured Applejack knew what she was doing.

Just as soon as she’d caught her breath, Applejack dove right back down. Mac groaned and shifted atop his fat as she drank, growing fatter and fatter all the while. By the time she pulled away next, she’d have a thick, wobbling flank, the beginnings of a second chin, and a belly that drooped down almost to her knees.

But by the time she next pulled away, neither her nor Mac would care.